1. Field of the Invention
The present invention relates to chromate treatment of surface treated steel sheets and strips (herein called steel sheets), which chromate treatment comprises forming a complex chromate film or layer composed of a cathodic electrolytic film and an anodic electrolytic film, excellent in corrosion resistance, paintability and, in particular, adaptability to cation electro-deposition paint coating, on the surface of galvanized or zinc alloy coated steel sheets, aluminium or aluminium alloy coated steel sheets and lead or lead alloy coated steel sheets.
2. Description of the Related Art
As for the electrolytic chromate treatment of galvanized steel sheets, it is known to perform a cathodic treatment in a treatment solution of CrO.sub.3 --H.sub.2 SO.sub.4, as disclosed in Japanese Patent Publication Sho No. 47-44417, and to perform a cathodic treatment in a treatment solution of CrO.sub.3 -SiO.sub.2 as disclosed in Japanese Laid-Open Patent Application Sho No. 60-110896. As for similar treatments for steel sheets coated with Zn, Al and Sn and their alloys, a cathodic electrolytic treatment in a treatment solution composed of CrO.sub.3, various metal ions, and fluorine or chlorine is disclosed in Japanese Patent Publication Sho No. 49-14457 and so on. All of these prior methods form a chromate film on metal or alloy coated steel sheets by a cathodic treatment. Generally speaking, the cathodic electrolytic film is insufficient in corrosion resistance despite its good paintability. Therefore, in order to compensate for the insufficient corrosion resistance of the conventional cathodic electrolytic chromate film, trials have been made to form an increased amount of film (specifically to increase the amount of chromium deposition). However, the chromate film, when applied in a greater amount, will be colored, thus damaging the surface appearance of the coated steel sheets, and will degrade the paintability of the sheets. Therefore, it has hitherto been difficult to form a film excellent in all of the surface appearance, corrosion resistance, and paintability. Moreover, when galvanized steel sheets or the like are continuously treated by the prior art cathodic electrolytic treatment, Zn.sup.2+ ions will accumulate in the treating solution to cause problems such that the corrosion resistance of the coated steel sheets is markedly lowered.
Referring to the anodic electrolytic treatment, it is also possible to form a chromate film on metal or alloy coated steel sheets by an anodic electrolytic treatment, but the resultant film is very poor in corrosion resistance and paintability. Further, in the case of galvanized steel sheets in particular, disadvantages of the anodic electrolytic treatment are that the metal of the steel sheets being treated will dissolve into the treating solution according to Faraday's law, thus causing inconsistency in the solution, and prohibiting of a consistent commercial operation.